A waveguide type light control device, such as an optical modulator, is a key element in a high-speed optical communications system and an optical information processing system. The optical modulator includes, for example, an optical modulator that uses dielectric materials, such as LiNbO3 (LN), and an optical modulator that uses semiconductor materials, such as InP and GaAs. Among these optical modulators, expectations have been placed on a semiconductor optical modulator that can be integrated with other optical elements and other electronic circuits, such as lasers and optical amplifiers, and that can easily achieve a reduction in size and a reduction in voltage.
An electroabsorption optical modulator and a Mach-Zehnder type optical modulator are each known as a typical semiconductor optical modulator.
The electroabsorption optical modulator is an optical modulator that uses the effect of allowing an absorption edge to shift toward a long wavelength by applying an electric field, such as a Franz-Keldysh Effect in a bulk semiconductor or a Quantum Confined Stark Effect (QCSE) in a multi-quantum well structure.
The Mach-Zehnder type optical modulator is an optical modulator that uses the effect of causing a change in the refractive index by applying an electric field, such as an electro-optic effect (Pockels Effect) in a bulk semiconductor or a Quantum Confined Stark Effect in a multi-quantum well structure.
The electroabsorption optical modulator is considered as a promising optical modulator because of being small in power consumption, being small in size, and being unlike an LN modulator that causes a drift by DC voltage. However, the electroabsorption optical modulator is characterized in that the waveform of an optical signal that has been transmitted through an optical fiber is deteriorated owing to wavelength chirping caused when modulated.
In more detail, owing to wavelength chirping, the spectrum of an optical signal that has been modulated becomes wider than that of the optical signal that has not yet been modulated. When the optical signal that has been modulated is transmitted through an optical fiber, the waveform of the optical signal undergoes waveform deterioration by the effect of dispersion of the fiber medium. As a result, transmission characteristics are deteriorated. The waveform deterioration becomes more noticeable in proportion to a rise in transmission speed and an increase in transmission distance.
On the other hand, the Mach-Zehnder type optical modulator can remove wavelength chirping in principle and is therefore expected to be a modulator used for ultra high-speed and long-distance communications.
For example, a semiconductor Mach-Zehnder type optical modulator is described in C. Rolland et al., 10 Gbit/s, 1.56 μm multiquantum well InP/InGaAsP Mach-Zehnder optical modulator, Electron Lett., Volume 29, 5th issue, pp. 471-472, 1993 (hereinafter, referred to as Document 1). This modulator is a lumped-constant type modulator that has a pin structure. In the lumped-constant type modulator having a pin structure, light is guided through a non-doped Multi-Quantum Well (MQW) region that is placed between a p-type semiconductor layer and an n-type semiconductor layer and that has a layer thickness of 0.4 μm. Accordingly, the light undergoes refractive-index modulation by an electric field with high efficiency. Therefore, the length of a phase-modulating portion can be extremely shortened. For example, the length of a phase-modulating portion of an LN modulator is 20 to 30 mm, whereas that of the lumped-constant type modulator can be set at 600 μm.
However, the lumped-constant type modulator has a great optical loss in the p-type semiconductor part. For example, the total insertion loss is 13 dB. Additionally, the lumped-constant type modulator has difficulty in performing an operation at 10 Gbit/s or more because of speed restrictions by the CR time constant.
FIG. 13 is a schematic sectional view of a waveguide of a Mach-Zehnder type optical modulator having a traveling-wave-type electrode structure, and shows a cross-sectional structure of an electric-field-applied portion. The Mach-Zehnder type optical modulator having the structure of FIG. 13 is a traveling-wave-electrode-type modulator that uses a Schottky electrode. This modulator is currently being researched thoroughly in order to solve the problem of the lumped-constant type modulator mentioned above, and is described, for example, in R. Spickermann et al., GaAs/AlGaAs electro-optic modulator with bandwidth>40 GHz, Electron Lett., Volume 31, 11th issue, pp. 915-916, 1995 (hereinafter, referred to as Document 2).
As shown in FIG. 13, the electric-field-applied portion is made up of an SI (Semi-Insulate)-InP cladding layer 71, an optical waveguide core layer 72 laminated on the SI-InP cladding layer 71, a ridge-shaped SI-InP cladding layer 73, a ground electrode 74, and a Schottky electrode 75 on the surface of the ridge. The SI-InP cladding layers 71 and 73 can be replaced by i (non-doped)-InP, and can be formed not only with InP but also with GaAs-based materials.
The conventional lumped constant type modulator having the pin structure has proved difficult in realizing a traveling-wave-type electrode structure because of the waveguide loss of an electric signal in the p-type electrode and because of mismatching in velocity between light and an electric field caused by the capacity component of the pin structure.
The Mach-Zehnder type optical modulator having the structure shown in FIG. 13 has realized a traveling-wave-type electrode structure by using a Schottky electrode. Additionally, this traveling-wave-electrode-type modulator can remove the defect described with reference to the lumped-constant type modulator by using an SI layer or a non-doped layer as a semiconductor.
However, in the Mach-Zehnder type optical modulator having the structure of FIG. 13, the distance GAP between the Schottky electrode 75 and the ground electrode 74 becomes equal to about 9 μm at a minimum, which can be regarded as a relatively great value, resulting from processing restraints. Therefore, the electric field strength (which is shown by arrows in FIG. 13) of the optical waveguide core layer 72 becomes small. As a result, the modulation efficiency of the refractive index of the optical modulator is lowered.
The modulation efficiency is small in the Mach-Zehnder type optical modulator having the structure of FIG. 13, and therefore, in order to perform a sufficient phase modulation, the phase-modulating portion is required to be lengthened, or a high operating voltage is needed. As a result, it is known that the traveling-wave-electrode-type modulator cannot be made as compact as the lumped constant type modulator (for example, about 10 mm) or has a high operating voltage (for example, Vπ=28V).
A modulator disclosed in U.S. Pat. No. 5,647,029 (hereinafter, referred to as Document 5) is known as another prior art example of the semiconductor optical modulator of the traveling-wave-type electrode structure. FIG. 14 is a cross-sectional view of a waveguide of a semiconductor optical modulator shown in Document 5. As shown in FIG. 14, the optical modulator 80 is a high-mesa waveguide type modulator in which an n-type InAlAs lower cladding layer 82, an optical waveguide core layer 83 including a quantum well, and an n-type InAlAs upper cladding layer 84 are laminated on an SI-InP substrate 81 in this order.
The semiconductor optical modulator shown in FIG. 14 is characterized in that the upper and lower surfaces of the optical waveguide core layer 83 are sandwiched between the n-type InAlAs cladding layers 82 and 84, and voltage is applied between the cladding layers 82 and 84 through electrodes 85 and 86.
The semiconductor optical modulator shown in FIG. 14 is additionally characterized in that high-speed optical modulation in which a driving frequency band reaches 40 GHz is realized by changing the distance “s” between the electrode 85 and the optical waveguide core layer 83 of the semiconductor optical waveguide or the thickness “t” of the cladding layer 82 between the optical waveguide core layer 83 and the SI-InP substrate 81 and by satisfying an impedance-matching condition and a velocity-matching condition between signal light and an electric signal.
However, since the structure of the semiconductor optical modulator shown in FIG. 14 does not have a potential barrier, plenty of electric current flows when voltage is applied. Therefore, this element is formed on the premise that a BRAQWET layer (Barrier-Reservoir And Quantum-Well Electron-Transfer layer) is used as the optical waveguide core layer 83. The BRAQWET layer is described in detail, for example, in T. Y. Chang et al., Novel modulator structure permitting synchronous band filling of multiple quantum wells and extremely large phase shifts, Electron Device Meeting 1989, Technical Digest, International, 3-6 Dec. 1989, pp. 737-740 (hereinafter, referred to as Document 3).
The BRAQWET layer has a structure in which an n-type semiconductor layer, an MQW optical waveguide core layer, a p-type semiconductor layer, and an n-type semiconductor layer are sequentially laminated. FIG. 15A and FIG. 15B each show a banded structure of the BRAQWET layer. FIG. 15A shows a state in which voltage is not applied, and FIG. 15B shows a state in which voltage is applied. As shown in FIG. 15A, the banded structure of the BRAQWET layer is a structure using a p-type semiconductor part as a potential barrier with respect to electrons by use of a difference in the Fermi level between the n-type semiconductor part and the p-type semiconductor part. As shown in FIG. 15B, since there is a barrier by which electrons are blocked from flowing through two electrodes when voltage is applied, a structure that permits voltage application to the optical waveguide is formed.
This structure is characterized by using the band filling effect caused by injecting electrons into the MQW optical waveguide core layer, that is, using a change in absorption coefficient or a change in refractive index. Only the electrons are injected into the MQW optical waveguide core layer, and holes do not contribute to a response produced when voltage is applied. The BRAQWET layer can respond to a high-speed electric signal since it does not operate through a hole that is small in mobility.
In practice, a lift of a band gap in the p-type semiconductor part effectively blocks an electric current, and therefore the n-type semiconductor and the p-type semiconductor are required to undergo an extremely precise concentration control operation. However, it is difficult to steeply control an n-type carrier concentration and a p-type carrier concentration at a layer interface. Additionally, if anon-doped area becomes large, an electric field is applied to this non-doped area, and the efficiency of the electro-optic effect will be lowered.
Therefore, it is very difficult to produce a practical optical modulator that employs the BRAQWET structure. In the optical modulator of each document mentioned above, the band gap is great, and the barrier is heightened by using the p-type semiconductor as a barrier layer. An optical modulator that has the thus formed structure and that has optical extinction characteristics sufficient to be usable in practice has so far been unknown.
A traveling-wave-type electrode photodiode that has across-sectional structure shown in FIG. 16 can be mentioned as an example in which the upper and lower parts of an optical waveguide core layer are sandwiched between n-type cladding layers, which is not an example of the optical modulator [Jin-Wei Shi and Chi-Kuang Sun, Design and Analysis of Long Absorption-Length Traveling-Wave Photodetectors, Journal of Lightwave Technology, December, 2000, Volume 18, 12th issue, pp. 2176-2187 (hereinafter, referred to as Document 4)]. A traveling-wave-type electrode photodiode 90 shown in FIG. 16 has a layered structure in which an n-type cladding layer 92, an optical waveguide core layer 93, an n-type cladding layer 94 are laminated on an SI-GaAs substrate 91. The optical waveguide core layer 93 sandwiched between the n-type cladding layer 92 and the n-type cladding layer 94 uses high-resistance GaAs (LTG-GaAs) in order to control an electric current generated when voltage is applied. The traveling-wave-type electrode photodiode 90 comprises an electrode 96 placed on the n-type AlGaAs cladding layer 94 and an electrode 95 placed on the n-type AlGaAs cladding layer 92.
However, since low-temperature growth GaAs (LTG-GaAs) is used as high-resistance GaAs, optical losses are generated by a fault caused by the low-temperature growth.
As described above, among the semiconductor Mach-Zehnder type optical modulators being presently researched, the lumped-constant type modulator proves difficult in operating at 10 Gbit/s or more because the optical loss is great in the p-type semiconductor part, and speed restrictions by the CR time constant are imposed. The traveling-wave-electrode-type modulator has a problem in the fact that the modulation efficiency of the refractive index is small, and the phase-modulating portion cannot be easily reduced in size, so that an operating voltage becomes high.